Homework Number 2 Solutions

Transcription

1 COMP311: Fall 2010 Ghulam Murtaza Homework Number 2 Solutions Question 1: Router Queue We consider the queue (or buffer) at an outgoing link of a router At the input of this queue, a train of 100 bits arrives at a rate of 1000 bits/sec at every one second interval The outgoing link from the router is operating at a rate of 500 bits/sec What is the average queue occupancy? Assume that there is no store-and-forward mechanism and the bits are transmitted as they are received Hint: It might be easier to solve this problem by drawing the cumulative arrival and departure processes, as shown in the class Solution: Without loss of generality, we assume that the first train of 100 bits arrives at time t = 0 We are also given that the router sends the bits as they are being received without using store-and-forward Then, the cumulative arrival and departure processes, A(t) and D(t), respectively, are shown in the following figure The figure also shows the queue size Q(t), the difference between A(t) and D(t) cumulative bits 200 A(t) D(t) queue occupancy (bits) Q(t)=A(t)-D(t) time (seconds) time (seconds) Figure 1: Cumulative arrival and departure processes (left) and the Queue size (right) We note that Q(t) follows a one-second cycle and in the first cycle (0-1 sec), 1000t 500t 01 t 02 Q(t) = t 01 t t 1 (1) From this expression, the average queue occupancy is Q = = 5 bits

2 Question 3: Persistence in HTTP You are running a web-server on your home machine that connects to the network service provider through a DSL link with 100 Kbps (kilo bits per second) connection speed You come to the University and try to access a web page being served by the web-server running on your home machine The campus network in the University runs as 100 Mbps with an access link of 8 Mbps Assume that the ISP networks run at very high speeds and flow control is not needed Further assume that the RTT from your University campus to your home is 150 ms The base html file size you request is 10 KB (10 kilo bytes) This base file also includes 3 small images The first image that appears on the page is 20 KB, the second which appears somewhere in the middle of the page is 30 KB and the third image, towards the end of the page, is 15 KB Requests for these images are sent by your browser in that order, without using any parallel TCP connections Neglect the time spent in local processing and parsing of html files and processing of images by the browser (a) What is the total number of HTTP requests generated from the browser on campus computer? Solution: 4 HTTP requests (there are only four objects) (b) How long would it take your browser from sending the first http request to receiving the base html file? Solution: 150ms + 150ms ms = 300ms + 800ms = 1100ms (c) Assume http/10 (non-persistent) How long would it take your browser from sending the request to displaying the complete fi le Solution: 4(150ms + 150ms) ms = 1200ms ms = 7200ms (d) Next consider a persistent http without pipelining How long would it take your browser from sending the request to displaying the complete f i le Solution: 150ms+4 150ms ms=750ms+6000ms=6750ms

3 (e) Finally consider a persistent http with pipelining How long would it take your browser from sending the request to displaying the complete f i le Solution: 150ms+150ms ms+150ms+75times8 =6450ms 100 Additional Questions From Chapter-2 Review Questions 3, 5, 6, 10 Problems 2, 4, 6, 7 Discussion Questions 5, 8 Solutions: See following pages!

4 Homework 2 Solutions Chapter 2 Review Questions 3 With Instant Messaging, typically the initiator contacts a centralized server to locate the address (IP address) of the receiver: client server model After this, the instant messaging can be peer to peer the messages between the two communicating parties are sent directly between them 5 No As stated in the text, all communication sessions have a client side and a server side In a P2P file-sharing application, the peer that is receiving a file is typically the client and the peer that is sending the file is typically the server 6 The IP address of the destination host and the port number of the destination socket 10 The applications associated with those protocols require that all application data be received in the correct order and without gaps TCP provides this service whereas UDP does not Chapter 2 Problems Problem 2 Access control commands: USER, PASS, ACT, CWD, CDUP, SMNT, REIN, QUIT Transfer parameter commands: PORT, PASV, TYPE STRU, MODE Service commands: RETR, STOR, STOU, APPE, ALLO, REST, RNFR, RNTO, ABOR, DELE, RMD, MRD, PWD, LIST, NLST, SITE, SYST, STAT, HELP, NOOP Problem 4 Application layer protocols: DNS and HTTP Transport layer protocols: UDP for DNS; TCP for HTTP Problem 6 The total amount of time to get the IP address is + RTT 2 Once the IP address is known, RTT O elapses to set up the TCP connection and another RTT O elapses to request and receive the small object The total response time is 2RTT RTT 2

5 Homework 2 Solutions Problem 7 a) b) c) +3 2RTT 0 = 8RTT 0 + +L+ RTT n = 4RTT 0 + +RTT 0 = 3RTT 0 + +L+ RTT n